<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
<link rel="stylesheet" href="commons.css"/>
</head>
<body>
<h1 class="c1">样式写在css文件中</h1>
<h2 class="c2">样式写在css文件中</h2>
<h3 class="c3">样式写在css文件中</h3>
</body>
</html>